1. Field of the Invention
The present invention relates generally to the field of fiber optic networks and more specifically to a system and method for tunable chromatic dispersion compensation.
2. Description of the Related Art
A typical fiber optic communication system includes transmission nodes, line nodes, and links. Transmission nodes are terminal nodes that include optical signal sources, such as a laser. Line nodes typically include optical amplifiers, couplers, and decouplers. In many common applications, the line nodes also include other devices, such as chromatic dispersion compensators. Links are used to convey optical signals between nodes.
Lasers that transmit optical signals on links, such as optical fibers, provide a narrow spectrum of light (i.e., a light pulse) that includes many wavelengths. Chromatic dispersion is a variation in the velocity of this signal according to wavelength. Among other things, this variation in velocity causes the light pulses of an optical signal to broaden as they travel through the link. This phenomenon, known as “pulse spreading,” can cause increased bit error rates if the light pulses spread to a point where they begin to overlap with one another. Chromatic dispersion increases linearly with distance traveled in the link. A tunable chromatic dispersion compensator is settable (with a finite resolution) and can be used to compensate for chromatic dispersion, without knowing the actual link length, using chromatic dispersion measurements.
One approach to measuring chromatic dispersion uses optical supervisory channel (OSC) reference signals of a link for measuring and mapping phase delay as a function of wavelength after propagating the reference signals through the link. For example, an OSC laser can be used to add reference signals at each transmitter and line node. After propagating through a link, the OSC signals are decoupled from the data signals and then used to determine the chromatic dispersion of the link. Subsequently, new OSC signals are coupled to the data signals and propagated to the next node. This approach measures the chromatic dispersion between a pair of adjacent nodes (i.e., across one link) within the system and uses this measurement to set the tunable chromatic dispersion compensator within the downstream node of the pair of adjacent nodes.
One drawback to this approach, however, is that dispersion compensators have finite resolution, which may result in residual dispersion that is not corrected. If there is residual dispersion for each link in the system, then the residual dispersion may accumulate downstream in the system. Another drawback is that OSC lasers typically have poorly controlled wavelengths, leading to general inaccuracies. This problem can be addressed by using more precise lasers at each node, but such a solution is expensive and does not necessarily ensure that there will be no residual dispersion. Again, if any residual dispersion exists at the nodes, then the dispersion accumulation problem may still persist, increasing bit rate errors.
As the foregoing illustrates, what is needed in the art is a more effective technique for compensating for chromatic dispersion that occurs between and across the nodes of a multi-node fiber optic communication system.